Radio frequency (RF) receivers are used in a wide variety of applications such as television receivers, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, satellite radio receivers, and the like. One common type of RF receiver is the so-called superheterodyne receiver. A superheterodyne receiver mixes the desired data-carrying signal with the output of tunable oscillator to produce an output at a generally fixed intermediate frequency (IF). The fixed IF signal can then be conveniently filtered and converted down to baseband for further processing. Thus a superheterodyne receiver requires two mixing steps.
Traditionally, certain RF receivers have adopted standard IFs. For example a television receiver translates a selected channel in the band of 48 MHz to 870 MHz to a standard IF of 44 MHz. Within the United States, FM radios typically translate FM audio signals, which are broadcast in 200 KHz channels in the frequency band from 88.1 MHz to 107.9 MHz, to a standard IF of 10.7 MHz. More recently, RF receivers have adopted low intermediate frequency (LIF) and zero intermediate frequency (ZIF) architectures to take advantage of processing capabilities of modern digital signal processors (DSPs).
Moreover high quality RF receivers use automatic gain control (AGC) circuits to adjust the gain or attenuation of various elements in the receiver in order to regulate the power levels. For example, a television signal with low input power can be amplified to increase the signal strength for further processing. In another example, a filtered signal may be too powerful for a following component, and so the filtered signal is attenuated to decrease the power level. Without such AGC circuits, the quality of the received desired signal would be reduced. For instance, the displayed image of a television signal would get dimmer as the power level dropped and eventually would start showing an increasing level of background noise. Conversely, the displayed image would be brighter as the power level rose and eventually would show image artifacts due to the system's non-linearities, like beat frequency waves or images in the background of the desired image.
Terrestrial and cable television transmission environments make AGC difficult due to the presence of blockers. A blocker is an unwanted channel with significant signal energy whose frequency is close to the desired channel frequency and thus is difficult to filter out. Since the blocker is not easily filtered, it can degrade the signal quality of the desired channel. Filtering out the undesirable energy of a blocker is especially difficult when the receiver uses an LIF or ZIF architecture because television transmission systems use many closely spaced channels.
Moreover the strongest blocker will sometimes be adjacent in frequency to the desired channel, and at other times be more remote in frequency. Also the blocker may have a much larger signal strength than the desired channel, and the signal strength can vary over time, for example, when a moving receiver passes into a tunnel or behind a building, or an obstruction, such as an airplane, passes between the transmitter and the receiver. These factors make AGC in LIF or ZIF signal processors especially difficult.
What is needed, then, are new analog baseband processor architectures for applications such as television receivers with AGC suitable for use in the presence of strong blockers and which are also suitable for LIF and ZIF architectures.
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